The invention relates to pistons used in internal combustion engines and more particularly to pistons which provide for variable compression ratios in such engines.
Internal combustion engines are known to achieve greater efficiency and performance with a high compression ratio. High compression engines do have a disadvantage, however. When operated at open throttle the high pressure created within the combustion chamber of the cylinder after ignition tends to cause a secondary post-ignition explosion. This condition is commonly known as "knock." In engines having a fixed compression ratio, knock is prevented at open throttle by retarding the spark or using a higher octane fuel, or by various other techniques. The use of higher octane fuel increases fuel costs and the other techniques decrease engine efficiency.
Low compression engines suffer far less from knock at open throttle even when burning lower octane fuel, such as unleaded fuel. When throttled however, low compression engines are less efficient than high compression engines and tend to achieve less complete combustion of fuel thereby emitting more pollutants in the exhaust.
It is therefore disirable to have an engine with a variable compression ratio which is high when the engine is throttled and decreases as the throttle is opened. Such an engine would be able to achieve high efficiency when throttled while preventing knock when heavily loaded without the need for using high octane fuel.
Several prior art systems have sought to achieve a practical variable compression ratio engine. One such system using a variable ratio piston is shown in U.S. Pat. No. 2,323,742. That system employs a 2-piece piston having an intermediate coil spring. The top and bottom of the piston are held together by a central connecter which is attached by a slip fit to the wrist pin holding the crank connecting rod. The top is secured by a threaded bolt. The numerous parts and the various slidable fittings shown in U.S. Pat. No. 2,323,742 are unsuited to present day automobile engines. The piston head is so heavy that the pressure in the combustion chamber will not overcome the inertia generated, thereby cancelling the benefits of the design. Furthermore, the many parts make the piston unreliable in an engine running at thousands of r.p.m. where the parts are subjected to such stresses as to cause failure and separation. The loosening or failure of any part within such a piston would severely damage or destroy the engine. Similar complex pistons are shown in U.S. Pat. No. 2,376,214 and U.S. Pat. No. 3,311,096.
Another method of adjusting the compression ratio of an engine is to provide auxiliary pistons or chambers which alter the size of the combustion chamber. Representative examples of such systems are shown in U.S. Pat. Nos. 2,215,986 and 2,260,982. These systems are highly complex, requiring complete redesign of the engine block, and cannot be retrofitted into present automobile engines.